I just received a sensor from you and it has an "F" on it, can I use it on the rear?

The marking on the sensors are just for your reference only. You can use on any tire

por la compra de 1 vienen los 2 sensores? el delantero y el trasero. saludos

Esta lista es para un solo sensor de reemplazo.

It will works with the SRTP 400 series?

Yes, will pair with Srtp400

How do I know if I have an older or newer generation SRTP300?

take a look at the monitor (or sensors). You will find the serial number under the model number. If your serial number ends with "Z" at the end, you have new generation. Otherwise your unit is older generation. New generation have been shipping since 2021.

will this sensor work on the tpms for a trike?>

No, will not match. Please go to www.sykik.com and search for SRTP670. Parts for this model as well as replacement sensors will show up.
In case you have a SRTP630 (with no display), you can search for SRTP630

Will the sensors pair automatically or do you have to pair them? I have a bad sensor on my front tire and already changed the batteries, it will only display dashes for the front sensor.

The sensors comes already paired in the box. But they could loose connection. You can try pairing them again. Also try resetting:
Turn off the display, remove the sensors. Turn the display on and then re-install the sensors.